Demand on a high speed of an optical network is rapidly increased as voice and text oriented services have evolved to video based services due to a rapid expansion of Internet. An average bi-directional transmission bandwidth to be guaranteed to a subscriber in a future optical network is required to be 100 Mb/s or more and a transmission distance must satisfy 20 km or more which is an established international standard based on a distance between a central office and a subscriber.
However, in the existing copper-wire based optical network, there is a limit in bandwidth available for each subscriber. Accordingly, an FTTH (Fiber-To-The-Home) is the only single possible alternative which can provide services by installing a single mode optical fiber to a subscriber, in order to meet the above requirements. Among various methods of building an FTTH, a WDM-PON has been perceived to be an ultimate solution where it is possible to provide subscribers with a bandwidth of 100 Mb/s or more like a bandwidth obtained in a point-to-point connection case while saving amount of using optical fiber and to guarantee a high quality of service (QoS).
In a WDM-PON, because separate wavelengths are used for respective subscribers when communicating with a central office (CO), each subscribers and the central office respectively must have a light source which emits light at a predetermined wavelength. Further, a WDM-PON also has a problem of maintaining wavelengths assigned to respective subscribers. That is, the wavelength of a light source positioned at a CO must correspond to a transmission wavelength of a wavelength-division multiplexing (WDM) filter of the CO and a transmission wavelength of a WDM filter positioned at a remote node (RN). In addition, the wavelength of a light source positioned at an optical network termination (ONT) must correspond to a transmission wavelength of a WDM filter of the CO and a transmission wavelength of a WDM filter of the RN.
Generally, an arrayed waveguide grating (AWG) is used as a WDM filter. In the meanwhile, the wavelength of a light source may vary as ambient temperature varies and a transmission wavelength of a WDM filter also varies accordingly. Further, the installation positions of a light source and a WDM filter are different, each light source and a WDM filter undergo different changes of temperature. In addition, the change rates of wavelength characteristics for a light source and a WDM filter are different depending on temperature. Accordingly, there has been a problem that it is required to monitor, control and manage, etc. for a wavelength alignment between a temperature-sensitive light source and other optical components in a WDM-PON. In order to solve the above problem, a light source which operates wavelength-independently is necessarily required and the essence in WDM-PON is to embody a low-cost or cost-effective light source which operates wavelength-independently.
Among wavelength-independently operating light sources that have been suggested currently, it is recognized that a wavelength-locked F-P LD is the most low-cost or cost-effective light source among WDM-PONs.
As illustrated in FIG. 1, a wavelength-locked FP LD is comprised of a broadband light source (BLS) 30; an optical circulator 40 for separating light being injected through an optical fiber and wavelength-locked output light; a WDM filter 50 for multiplexing the light being injected; and an F-P LD 60 being oscillated in a multi-mode. When the BLS 30 is injected into the F-P LD 60 being oscillated in a multi-mode after passing through the WDM-filter 50, the F-P LD 60 oscillates in a quasi-single mode and is wavelength-locked by the injected light. Thus, adjacent modes are suppressed and mode partition noise is reduced. In addition, output light from the wavelength-locked F-P LD 60 passes the WDM filter 50 and is transmitted to a receiving end (not shown) of a CO.
FIG. 2 illustrates a WDM-PON system which employs a wavelength-locked F-P LD in accordance with a prior art. Referring to FIG. 2, a WDM-PON system in accordance with a prior art is comprised of a CO 100, a RN 200, and a plurality of ONTs 300. An AWG is employed as a first WDM filter 120a which is used at the CO 100 and a second WDM filter 210a which is used at the RN 200, respectively. The CO 100 is comprised of a BLS 130a, a first WDM filter 120a, and n-numbered optical transceivers 110a, . . . , 110n. The plurality of ONTs 300 is connected to the RN 200 through a single-mode optical fiber. A wavelength-locked optical signal outputted from the CO 100 is de-multiplexed by a second WDM filter 210a of the RN 200 and is transmitted to receiving ends of respective ONTs 300. Light spectrum generated from the BLS 130a is flat. Light generated from the BLS 130a is filtered by the first WDM filter 120a of the CO 100 and is injected into respective F-P LDs 110a, . . . , 110n of the CO 100. Also, light generated from the BLS 130a is filtered by the second WDM filter 210a of the RN 200 and is injected into respective F-P LDs 300a, . . . , 300n of the plurality of ONTs 300. The respective F-P LDs 110a, . . . , 110n of the CO 100 and the respective F-P LDs 300a, . . . , 300n of the plurality of ONTs 300 are oscillated in a quasi-single mode and are wavelength-locked by injected light. Respective up-stream optical signals outputted from respective wavelength-locked F-P LDs 300a, . . . , 300n and respective down-stream optical signals outputted from respective wavelength-locked F-P LDs 110a, . . . , 110n undergo a filtering effect during respective multiplexing processes by the second WDM filter 210a of the RN 200 and the first WDM filter 120a of the CO 100. Then, the respective up-stream optical signals and the respective downstream optical signals respectively pass a single-mode optical fiber and undergo again a filtering effect during respective de-multiplexing processes by the first WDM filter 120a of the CO 100 and the second WDM filter 210a of the RN 200. Those filtering effects have a problem in that they cause an increase in mode partition noise of a wavelength-locked F-P LD and thus cause degradation of a noise characteristic. Further, if injected light is positioned between oscillation modes of the F-P LDs 110a, . . . , 110n and 300a, . . . , 300n, degradation in noise characteristic more increases because two adjacent modes of an F-P LD 60 are selected depending on a bandwidth being injected and more optical power is lost by filtering. Therefore, the degradation in noise characteristic due to filtering causes a low performance of a WDM-PON employing wavelength-locked F-P LDs 110a, . . . , 110n and 300a, . . . , 300n, and also becomes an obstacle of wavelength-independence.